chapter 10 our starn00006757/astronomylectures... · (sun, stars) two kinds of nuclear reactions. 7...

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Chapter 10Our Star

X-ray visible

Radius:6.9 × 108 m (109 times Earth)

Luminosity:3.8 × 1026 watts(more than our entire world uses in 1 year!)

Mass:2 × 1030 kg (300,000 Earths)

Why does the Sun shine?

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Is it on FIRE?

Is it on FIRE?

Luminosity (J/s = W)~ 10,000 years

Chemical Energy Content (J)

Is it on FIRE? … NO

Luminosity~ 10,000 years

Chemical Energy Content

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Is it CONTRACTING?

Luminosity

Gravitational Potential Energy

Is it CONTRACTING?

~ 25 million years

Luminosity

Gravitational Potential Energy

Is it CONTRACTING? … NO

~ 25 million years

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It is powered by NUCLEAR ENERGY!

Luminosity~ 10 billion years

Nuclear Potential Energy (core)

E = mc2

—Einstein, 1905

It is powered by NUCLEAR ENERGY!

E = mc2

—Einstein, 1905

•Nuclear reactions generate the Sun’s heat•But they require very high temperatures to begin with•Where do those temperatures come from?•They come from GRAVITY!

•The tremendous weight of the Sun’s upper layers compresses interior

•The intense compression generates temperatures >107 K in the innermost core

•And that’s where the nuclear reactions are

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•The compression inside the Sun generates temperatures that allow fusion

•The fusion reactions in turn generate outward pressure that balances the inward crush of gravity

•The Sun is in a balance between outward pressure from fusion and inward pressure from gravity

•This is called gravitational equilibrium

Gravitational Equilibrium

The Sun’s Structure

Solar wind:A flow of charged particles from the surface of the Sun

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Core:Energy generated by nuclear fusionTemps to 15 million K

Radiation zone:Energy transported upward by photonsTemps to 10 million K

Convection zone:Energy transported upward by rising hot gasTemps to 1 million K

Photosphere:Visible surface of SunTemp ~ 6,000 K

Chromosphere:Middle layer of solar atmosphereTemp ~ 104–105 K

Corona:Outermost layer of solar atmosphere Temp ~1 million K

How does the Sun produce energy?

Big nucleus splits into smaller pieces

(Nuclear power plants)

Small nuclei stick together to make a bigger one

(Sun, stars)

Two kinds of nuclear reactions

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• The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus.

• The hydrogen nuclei (protons) have to touch for this to happen• For them to touch requires temperatures > 10,000,000 K• Why does it have to be so hot?

• Why would that require high temperatures?

• This is the way the Sun produces its energy• But this is a summary reaction• The actual reaction—the proton-proton chain—is more complex

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• This is the complete reaction…the proton–proton chain• It’s interesting how long each of these steps takes…

109 years 1 second 10 6 years

IN4 protons

OUT4He nucleus

6 gamma raysnot two as shown here

Anybody know why?

2 positronsannihilate with two e-

to produce two γ rays

2 neutrinos

The total mass is 0.7% lower

The Sun’s energy comes from this…

E = mc2

• This reaction is very sensitive to temperature

• That sensitivity, together with gravitational equilibrium, makes

the “solar thermostat” possible…

Solar Thermostat

•Core temperature drop -> fusion rate drop

•Gravity crunch exceeds pressure push

•Core compression -> temperature rise

•Original conditions restored

•Core temperature rise -> fusion rate rise

•Pressure push exceeds gravity crunch

•Core expands -> temperature drops

•Original conditions restored

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How does the energy from fusion get out of the Sun?

• Gamma ray photons from the core bounce randomly through the radiation zone• This can take up to a million years…

• Plasma convection takes energy to the surface of the convection zone• This takes about a week• The surface of the convection zone is the visible surface of the Sun—the photosphere

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This is a close-up of the photosphere of the Sun

The brighter areas are where hot plasma reaches the photosphere

The darker areas are where cool plasma sinks back down

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This is called “granulation” because of the way it looks

Granulation is not static, and the “granules” are not small

from http://www.kis.uni-freiburg.de/~pnb/granmovtext1.html

Motion sped up ~600X Total actual elapsed time = 35 min

Average size of a convection cell ~1300 km (800 miles)

Speed of gas motion 1-2 km/s (2000-4000 mph)

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We’ve talked about the Sun’s structure

We’ve talked about how it generates energy

But how do we know we’re right?

• make mathematical and computational models of the Sun

• use them to predict things about the Sun, e.g.:

– predict the way the Sun vibrates based on hypotheses about

internal structure and composition

– predict the number of neutrinos the Sun produces based on

hypotheses about the nuclear reaction mechanism

• observe solar vibrations

• observe solar neutrinos

• compare observations to predictions

Does this process sound familiar?

THE SCIENTIFIC METHOD

This is how

•Patterns of vibration

on the surface tell us

about what the Sun is

like inside.

•Models of the

interior of the Sun are

adjusted until the

vibrations of the

model match the

observed vibrations of

the Sun

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• Here is a movie made from observations of the

vibrating surface of the Sun

From http://solarscience.msfc.nasa.gov/Helioseismology.shtml

• This is how we think the Sun

generates its energy

• But how can we test it?

• The proton-proton chain

produces a certain number

and type of neutrino

• Unlike gamma rays, neutrinos

don’t interact very much with

the Sun

• And they come straight out of the Sun to

Earth

• So looking at them is nearly as good as

looking right at the nuclear reactions going

on in the core

• In fact, they fly directly through

any normal matter

• There are trillions of them flying

through each of us right now

Solar neutrino problem:

• Early searches for solar neutrinos

found only ~1/3 of the predicted

number

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Solar neutrino problem:

• Early searches for solar neutrinos

found only ~1/3 of the predicted

number

• It took 20 years, until scientists

understood neutrino behavior

better, to figure out why

• On the way to Earth from the

Sun, the neutrinos changed form

from the type produced in the

proton-proton chain (νe) to the

other two types (νμ and ντ)

• When this is taken into account,

the observed number matches

predictions

Types of Solar Activity

• Sunspots

• Solar flares

• Solar prominences

• Coronal mass ejections

• All related to magnetic fields

• We’re going to focus on sunspots

Sunspots…

• Cooler than

other parts of

the Sun’s

surface

(4,000 K)

• Strong

magnetic

fields

• How do we

know?

• The Zeeman

Effect

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We can detect

magnetic fields in

sunspots by

observing the

splitting of spectral

lines

The Zeeman Effect

The Sun’s magnetic field is essentially

the same as an electromagnet’s

field…and a planet’s magnetic field

Charged particles circulate as an

electric current in looped paths

This generates the magnetic field

Convection of the plasma beneath the

surface of the Sun causes its field

Sunspots occur where field lines poke

out of the Sun’s surface…we’ll see why

in a bit…

And this keeps the sunspot cooler

than the surrounding plasma

Here’s how…

• Charged particles spiral along magnetic field lines

• Field lines “fence out” hot plasma

• Allows sunspots to exist for long periods…

How do magnetic fields make sunspots “cool”?

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• Loops of bright gas often connect sunspot pairs

• The gas is following magnetic field lines

• All of solar activity—sunspots, flares, etc—is tied to the Sun’s magnetic field

• The Sun’s magnetic field varies in a more or less regular way

• The most obvious manifestation of the variation is the number of sunspots

• The number of sunspots rises and falls in 11-year cycles.

• One way of tracking sunspot variation is to measure the percentage of the

Sun’s surface covered by sunspots

• All of solar activity—sunspots, flares, etc—is tied to the Sun’s magnetic field

• The Sun’s magnetic field varies in a more or less regular way

• The most obvious manifestation of the variation is the number of sunspots

• The number of sunspots rises and falls in 11-year cycles.

• One way of tracking sunspot variation is to measure the percentage of the

Sun’s surface covered by sunspots

• Another way is to look at where the sunspots are

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• The sunspot cycle has to do with the winding and twisting of the Sun’s magnetic field

• Think of the Sun’s magnetic field as lines of bar magnets end-to-end


• Think of the Sun’s magnetic field as lines of bar magnets end-to-end• The Sun’s equator rotates faster than the poles• This eventually breaks apart magnetic linkages (like breaking apart the

lines of bar magnets)


• Think of the Sun’s magnetic field as lines of bar magnets end-to-end• The Sun’s equator rotates faster than the poles• This eventually breaks apart magnetic linkages (like breaking apart the

lines of bar magnets)• Sunspots form where the broken magnetic fields poke out

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• The global magnetic field reorganizes itself at each solar minimum• But the polarity is opposite what it was at the previous minimum

• There are 11 years between maxima, but the full cycle lasts 22 years

min max min max min

Temperature

Luminosity

HERTZSPRUNG-RUSSELL

DIAGRAM

• A Hertzsprung-Russell diagram

plots luminosity versus surface

temperature or spectral type

• Remarkably, that’s all you need

to know about a star you are

interested in to have access to

a lot of information about it

• Once the star is on the H-R

diagram, you can find out if it’s

a giant, main sequence star, or

white dwarf

• You will have estimates of its

mass, radius, and main-

sequence lifetime (if it’s a main

sequence star, which most stars

are)

• How long stars stay on the

main-sequence depends on

their mass, which can vary

greatly from star to star

• Star mass affects how long it

takes to use up core hydrogen

• More massive stars don’t stay

as long on the main sequence

After the Main Sequence - Broken Thermostat

On the main sequence, the “core thermostat” works

But when core hydrogen is gone

it doesn’t work

• While a star is on the main sequence, it has a core thermostat, like our Sun’s solar thermostat

• It keeps the rate of H->He fusion constant• And that fusion supports the core and the star

through gravitation equilibrium• When hydrogen is used up, H->He fusion turns off• The core is now all helium, no hydrogen• The star is dying…

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After the Main Sequence - Broken Thermostat

But when core hydrogen is gone, the core thermostat can’t

function

• With no fusion energy, the He core contracts (is compressed) and heats

• The hot core acts like a stove burner, causing H to fuse to He in a shell around the core

• This produces even more outward pressure than on the main sequence, and the star swells up into a “red giant”

• But the burning shell doesn’t restore the “core thermostat”, because it is outside the core

• The He core continues to contract until the atoms are “touching” each other

• Meanwhile the hydrogen-burning shell continues to deposit He “ash” on the core, and it continues to heat up

• Eventually it gets hot enough (100 million K) for He to fuse to carbon

• It becomes a “helium-burning star…

• The helium burning star experiences something like a second main sequence• This is because the core thermostat is temporarily fixed due to He->C fusion in the core• There is also hydrogen still burning in a shell, but the shell is not as compressed• This is because the helium fusing in the core expands the core• This generates less energy than in the red giant, but more than on the main sequence• So helium burning stars are in between in size• But eventually, the helium gets used up just like the core hydrogen did (but more quickly)• This leaves behind a carbon core

Double-Shell Burning• After core helium fusion stops, the carbon core collapses (is compressed) and heats• Meanwhile hydrogen continues to burn in a shell around the carbon core, depositing

a shell of helium on the core• The shell of helium begins fusing to carbon while the hydrogen shell above it fuses to

helium• The star has become a red giant again, this time called a “double-shell burning star”• This double-shell-burning stage is unsteady, and the fusion rate periodically spikes

upward in a series of thermal pulses.• With each pulse, carbon gets dredged up from the core and transported into the

overlying “envelope”• Soon that carbon will enrich the interstellar medium…• The carbon continues to shrink and get hotter• But its atoms “touch” before it gets hot enough (600 million K) to fuse carbon

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Planetary Nebulae and White Dwarfs

• Double-shell burning ends with a pulse (or pulses) that ejects the gas envelope into space as a planetary nebula.

Life stages of a low-mass star like the Sun

• Double-shell burning ends with a pulse that ejects the gas envelope into space as a planetary nebula

• The core left behind becomes a white dwarf

• White dwarfs are inert balls of carbon and oxygen (from fusion of helium and carbon)

• They might also have a little residual H/He atmosphere


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• They are about the size of Earth






• They are about the size of Earth• Some even contain gigantic

diamonds!

For more information about the diamond star, seehttp://www.cfa.harvard.edu/news/archive/pr0407.html


For more information about the diamond star, seehttp://www.cfa.harvard.edu/news/archive/pr0407.html

• The planetary nebulae surrounding white dwarfs come in all shapes and sizes


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Life Stages of High-Mass Stars

• Main-sequence life of high-mass stars is similar to low-mass stars:— Hydrogen core fusion (main sequence), but faster

• High-mass main-sequence stars fuse H to He at much higher rates than low-mass stars

• This is partly because their higher mass means higher core temperatures

• But it’s also because they use carbon, nitrogen, and oxygen as catalysts.

• The “CNO cycle” is shown at left

CNO Cycle

Life Stages of High-Mass Stars

• Main-sequence life of high-mass stars is similar to low-mass stars:— Hydrogen core fusion (main sequence), but faster

• Early stages after main sequence are similar for high-mass stars and low-mass stars:— Hydrogen shell burning (as a supergiant)— Helium core fusion (as a supergiant)— But high-mass stars can go beyond that and do

“advanced nuclear burning”

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Advanced Nuclear Burning

• Advanced nuclear burning proceeds in a series of nested shells.

• It’s during this process—and afterwards—that stars synthesize most of the chemical elements

• If the star is massive enough (> ~8MSun), Tcore is high enough to make very large nuclei fuse

• In these stars, a variety of different fusion reactions in the nested shells can make elements all the way up to iron

• Why “energy per nuclear particle” to the right and “mass per nuclear particle” on this graph?

• Because mass and energy are equivalent:E = mc2

• But iron is a dead end for fusion because nuclear reactions involving iron do not release energy

• This is because iron has the lowest energy per nuclear particle of all elements

• Elements smaller than iron can fuse with a release of energy

• Elements larger than iron can fission with a release of energy

• But iron cannot fuse to anything larger, or fission to anything smaller

• So iron cannot generate any energy to support the core

• And when iron appears, the star’s death is imminent

• Advanced nuclear burning beyond that possible for a low mass star proceeds in a series of nested shells until iron appears

• Once iron appears, the dying star’s fate is sealed

• The iron can’t fuse, so the core collapses in a matter of milliseconds!

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Supernova Explosion

• Why does this cause a supernova?

• The collapse heats the core to the point that the iron nuclei dissociate into protons and neutrons

• The protons then combine with free electrons to produce more neutrons and neutrinos

• With the loss of the electrons, electron degeneracy pressure disappears

• The core collapses to a ball of neutrons, supported by neutrondegeneracy pressure

Supernova Remnant

• The energy released by the collapse of the core, the “bounce” when neutron degeneracy pressure kicks in, and the massive production of neutrinos drives outer layers into space

• It also drives synthesis of chemical elements up to uranium

• Left behind is either a neutron star or, if the core is massive enough to “break” the neutron degeneracy pressure, a black hole

• The outer envelope of the star moves out into space, forming a nebula

• The Crab Nebula is the remnant of the supernova seen in A.D. 1054

• A star’s mass determines its entire life story because it determines its core temperature.

• High-mass stars have short lives, eventually becoming hot enough to make iron, and end in supernova explosions.

• Low-mass stars have long lives, never become hot enough to fuse carbon nuclei, and end as white dwarfs.

chapter 10 our starn00006757/astronomylectures... · (sun, stars) two kinds of nuclear reactions. 7...

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